Periodontal status of maxillary central incisors after orthodontic traction: a longitudinal follow-up

Abstract The aim of this study was to evaluate the clinical and radiographic periodontal status of impacted permanent maxillary central incisors (Mx.1) after a long term of orthodontic traction. Methodology This split-mouth study evaluated a sample of 11 patients (five females, six males) treated with Mx.1 unilateral traction one to 28 years after the removal of orthodontic appliances. The traction Group (TG) consisted of 11 Mx.1 and the Comparison Group (CG) comprised 11 spontaneously erupted contralateral Mx.1. High-resolution CBCT exams of central incisors were performed using Accuitomo (J. Morita, Kyoto, Japan). Cross-section imagens passing through the center of maxillary central incisors were used to measure buccal and lingual alveolar bone level. Presence of fenestration, root dilacerations, root coverage, and position of the root apex were also assessed in the same images. Clinical parameters included periodontal probing depth, attachment level, gingival bleeding index, plaque index, degree of gingival recession, amount of gingival mucosa, and evaluation of interproximal papilla and black triangle. Digital model analysis included an assessment of clinical crown height and width. Intergroup comparisons were performed using paired t-, McNemar’s, and Wilcoxon tests (p<0.05). Results Compared to CG, we found a significantly thinner labial bone plate thickness in TG at the middle (p=0.000) and apical (p=0.009) root level. We also observed an apical displaced labial bone crest level in TG (p=0.000). The Traction Group showed a greater frequency of root dilacerations and gingival recessions, a decreased amount of keratinized mucosa, and a decreased clinical attachment level at the labial aspect compared to contralateral teeth. Conclusions A decreased thickness and height of labial alveolar bone and gingival recessions were found in maxillary central incisors 15 years after orthodontic traction. Though incisor traction might cause some periodontal impact, differences are acceptable under a clinical point of view considering the cost-benefit ratio.


Introduction
Impaction of maxillary permanent incisors has been found in the range of 0.2-1% of the population. 1 Moreover, the most frequent feature causing school bullying is the absence of anterior teeth. 2,3 In this perspective, permanent maxillary central incisor (Mx.1) retention has a psychosocial priority to be solved.
Clinically, eruption disturbances can be diagnosed when a six-month tooth eruption delay or more is observed compared to its homologue. 4,5 Eruptive delays occur due to obstructive or traumatic cause. [6][7][8][9] Obstructive factors involve any kind of physical barrier to the eruption, such as supernumerary teeth, tooth-bone discrepancies, gingival fibrosis, ankylosis, retained primary teeth, early loss of deciduous teeth, presence of cysts, odontomas or tumors in the region, among others. [10][11][12] Trauma can cause germ damage or a positional change of permanent teeth, which prevents spontaneous eruption. 8 Treatment management may include a broad range of options, including passive observation, surgical exposure and traction, and tooth extraction followed by prosthesis or lateral incisor substitution. 6,[13][14][15] Diagnosis and treatment planning in cases of impacted teeth require clinical and radiographic examination. Twodimensional (2D) radiographic images were the main instrument for examination. However, these exams could normally contain distortions, positioning errors, and tooth overlaps which impaired correct analyses.
CBCT (Cone Beam Computed Tomography) is the new gold standard, with a 3-dimensional (3D) parameter that enables multiple plane analyses for adequate diagnosis. 16 Tooth traction in a closed technique is the treatment usually indicated in the literature, requiring surgical exposure, attachment placement, and orthodontic movement. 6,7,9,[17][18][19][20] However, evidence is scarce on the long-term esthetic and periodontal aspects of such cases. Three previous studies used CBCT to evaluate impacted Mx.1 treated with closed-eruption technique followed by orthodontic traction. 21-23 Shi, et al. 22 (2015) evaluated root and alveolar bone status before and after traction and showed that, after treatment, the impacted incisor root showed the same stage of development compared to its contralateral and that both incisors had some alveolar bone loss, a thin alveolar bone surrounding the roots, and a pulp unaffected by traction. Sun, et al. 23 (2016) Table 2 includes the ages of each patient pre-, post-treatment, and at follow-up.
All patients were treated with the closed eruption technique. A mucoperiosteal flap was created to expose the impacted tooth and an orthodontic button with a stainless-steel ligature was bonded to the exposed surface of the incisor using a composite adhesive system. The direction of force was vertical toward the occlusal plan using 150g for all cases. The flap was sutured leaving the ligature wire emerging in the center of the alveolar ridge. Different traction modalities were employed. Seven patients used removable appliances with coil springs; one patient received rapid maxillary expansion and a coil spring incorporated to a Haas-type expander, and three patients underwent traction with fixed appliances.
During orthodontic traction, light forces were applied via elastomeric chains until the impacted tooth was exposed in the oral cavity. The attached button was Supernumerary teeth 2

Dental trauma
Horizontal 2

Vertical 3
Vertically inverted 3  All periodontal clinical measurements were performed at six sites including the mesial, center, and distal regions of labial and lingual crown surfaces.
Identification of the mucogingival junction was performed using Schiller's IKI solution staining. 26 To evaluate plaque index (P), all teeth were airdried and examined with a plaque disclosing agent

Digital models
Conventional dental models were obtained after clinical evaluation. Dental models were scanned using a 3Shape R700 3D scanner (3Shape A/S, Copenhagen,  Denmark). Clinical crown height was measured from the gingival margin to the incisal edge according to Andrews. Also, the width of both central incisors were evaluated using the OrthoAnalyzerTM 3D software (3Shape A/S, Copenhagen, Denmark). 27,28 Error study CBCT and dental models were measured twice by the same examiner within a month's interval. Random and systematic errors were estimated by comparing the first and second measurement with the Dahlberg formula and paired t-tests (p<5%), respectively.

Statistical Analyses
The Kolmogorov-Smirnov test showed normal

Results
Random errors for measurements performed on CBCT exams and digital dental models varied from 0.23 (lingual bone plate thickness in the apical third) to 0.35 mm (labial alveolar crest height). No significant systematic error was found.
Maxillary central incisors showed a significantly thinner labial bone plate in the middle and apical root levels in the Traction Group with a difference of approximately 0.5 mm (Table 3). A decreased labial alveolar crest height was observed in TG compared to CG with a mean difference of 2.3 mm ( Table 3).
The Traction Group showed greater gingival recession (difference of 0.45 mm), decreased amount of keratinized mucosa (difference of 0.9 mm), and a more apically displaced attachment level (difference of 0.66 mm) than the CG ( Table 3).
The experimental group showed significantly greater frequency of root dilacerations (72.7%) and smaller bone coverage on the root labial aspect compared to the CG (  (r=-0.64) was found between labial bone plate thickness and attachment level in the Traction Group (Table 5). A moderately positive correlation (r=0.59) was found between labial alveolar crest height and attachment level (Table 5). However, a small voxel size and small field of view was used in our study, contributing to a small study error.

Discussion
Results showed that, on average, 15 years later there was not a significant labial bone loss in the analyzed sample. TG showed both thinner labial alveolar bones and greater labial bone dehiscence than spontaneously erupted contralateral incisors (Table 3). Labial bone loss is a common complication of orthodontic traction, as shown in previous studies using high-resolution CBCT. 22,23 A previous study reported that impacted maxillary incisors showed reduced buccal bone height after treatment and that buccal bone loss is discontinued and remained stable two years after treatment. 21 In the sample, most of the impacted central incisors showed malposition before traction and, therefore, were more prone to labial bone dehiscence after traction. Surgical management of an impacted tooth is also a possible explanation for labial  bone loss. 19 Several surgical techniques are adequate for orthodontic traction. 6,7,9,[17][18][19][20] The technique that was used in this study was widely indicated in the literature, requiring surgical exposure, attachment placement, and closed eruption. 6,7,9,[17][18][19][20] During the exposure technique, a conservative removal of the surrounding bone to bond the traction attachment is necessary. Traction direction should also be controlled to move the incisors toward the center of the alveolar bone crest 36,37 . On the other hand, the lingual alveolar bone was similar in both groups, in accordance with previous studies. [21][22][23] Labial bone dehiscence is a risk factor for the development of gingival recessions. 13,38,39 TG showed more gingival recession on the labial aspect than CG ( Table 3). The amount of gingiva was also smaller in the Traction Group compared to its antimere ( Table   3). In this study, the mean gingival recession in the labial aspect found between the two groups was 0.45 mm. A previous study found gingival recessions of 0.21 mm in maxillary incisors after orthodontic traction using a closed eruption technique. 40 Previous studies also reported that gingival recessions are often observed after orthodontic traction of maxillary central incisors. 13,41 This sample also showed a 73% ratio of gingival resection in TG and no correspondent answer was found in CG.
On the labial aspect, the Traction Group also showed an apically displaced attachment level compared to CG (Table 3). This result is in agreement with a previous study showing a decreased attached gingiva in patients treated with closed-eruption incisors. 20 Loss of attachment may be associated to orthodontic procedures and toothbrushing injuries. 42 In this study, attachment level was correlated with a thinner labial bone plate and with an increased labial alveolar crest height (Table 5). Labial dehiscence is a predisposing factor for loss of attachment. 39,43 The probing depth in TG was similar to CG. These findings are in agreement with a previous study and are probably explained by the development of a long connective attachment replacing labial bone loss. 20,39 No intergroup difference was found for gingival bleeding index, and clinical crown height and width, and no black spaces were found in both groups.
The Traction Group showed a greater frequency of root dilacerations than its antimeres (Table 4).
These findings are explained by trauma as the main etiological factor of incisor impaction in our sample (Table 1). Although root dilacerations were present, most of the root apex were within the limits of the alveolar ridge (Table 4). Previous studies analyzing impacted inverted maxillary central incisors also found many root dilacerations, Shi,et al. 22 (2015)